1. Field of the Invention
The present invention relates to an ultrasound diagnostic imaging apparatus.
2. Description of Related Art
Ultrasound diagnostic imaging apparatus is a medical imaging device which obtains tomographic images of soft tissues inside a subject from its body surface in a minimally invasive way by performing the ultrasound pulse echo technique. Such ultrasound diagnostic imaging apparatus has number of characteristics such as smaller in size, lower in cost and safer due to no exposure to radiation such as X-rays comparing to other medical imaging devices and such as blood flow imaging is possible by applying the Doppler effect. Therefore, such ultrasound diagnostic imaging apparatus has been used widely in the fields of circulatory system (coronary artery of heart), digestive system (stomach and intestines), internal medicine system (live, pancreas, spleen), urinary system (kidney, bladder), obstetrics and gynecology system and such like.
Moreover, in recent years, harmonic imaging (HI) diagnosis using harmonic component and not conventional fundamental component of ultrasound is performed in order to improve accuracy of diagnosis using ultrasound images. Harmonic component is a frequency component which is generated by the fundamental component to be transmitted being affected by nonlinearity of inside a subject. Because clear diagnostic images which cannot be obtained by the conventional B-mode diagnosis using fundamental component can be obtained, HI diagnosis is becoming the normal diagnosis modality.
The ultrasound images obtained by HI diagnosis have advantages such as improvement in contract resolution due to having high S/N because their sidelobe level is small and improvement in azimuth resolution due to narrow beam width comparing to ultrasound images of fundamental component. Further, there are number of advantages such that multiple reflections do not occur because sound pressure is small in short distance and fluctuation of sound pressure is small, that degradation beyond focus is about the same as in fundamental component and that great invasion depth can be obtained in harmonic component comparing to fundamental component of same frequency.
Although ultrasound diagnostic imaging using harmonic component has number of advantages as described above, there is a problem that distance resolution does not improves because the frequency band of ultrasound wave by which sending and receiving is carried out is limited to the frequency band defined by the characteristic of the ultrasound probe to be used.
In view of the above problem, JP 2004-298620 discloses performing phase adjustment of waveform with respect to two fundamental waves having difference frequencies to transmit and receive ultrasound wave and obtaining harmonic component of broadband by using a difference frequency component obtained from the received two fundamental components in a conventional ultrasound diagnostic imaging apparatus.
Moreover, JP 2008-43721 discloses transmitting ultrasound wave by a transmission signal having a waveform of broadband where the phase is adjusted and obtaining harmonic component of broadband by using the base-band harmonic component obtained by receiving the ultrasound wave.